This invention relates to metal shells used to form ends of can type containers. Most can type containers, for example beer cans and soft drink cans, are required to withstand internal pressure, rough handling, and substantial temperature differences, yet maintain a complete hermetic seal to protect the contents of the can. Cans of this type are used in very large volumes, billions of cans per year, and at present the metal most used for this purpose is aluminum due to its light weight, comparative inexpensiveness and workability.
The typical modern can consists of a unitary deep drawn body, usually with a necked inward throat at the top which terminates in an outwardly extending body curl, and an end for the can which comprises the shell (to which the present invention pertains) provided with self-opening structure such as tear tabs and related score lines in the shell. The shells are manufactured from sheet metal by severing a suitable blank from a strip thereof, forming the blank to define a central panel surrounded by a reinforcing countersink and chuck wall configuration, and a shell curl which is designed to interact with the body curl in seaming apparatus to attach the end to the can with the requisite hermetic seal. In most instances the underside of the shell or end curl is provided with a sealing compound to assist in the formation of the seal.
The shell is the basic part of the end and is formed from the blanks, then the shells are operated upon in converting apparatus which adds the desired score lines, tear tab, and the integral rivet attachment between the shell and the tab, all in known manner. The sealing compound may be applied to the underside of the shell, specifically to the downward facing or bottom portion of the shell curl, either before the converting operation, or after, the former being more typical.
One of the major endeavors of designers of can ends is to provide a shell of as thin material as is possible, since this can result in substantial savings of material, and therefore expense. However the integrity of the shell, and its ability to withstand buckling from internal pressures in particular, imposes restrictions upon the use of very thin material in the shell formation. The ability of the thin metal to withstand the drawing and working imposed upon the blank during the formation of the shell generally calls for use of somewhat thicker metal, in order to accommodate thinning in the region where the reinforcing structure is formed in the shell.
In typical prior art operations for the forming of shells, a blank is severed from sheet material, usually steel or aluminum, and it is then formed to a shape comprising a generally flat central panel and a chuck wall extending, in this initial stage, upwardly and outwardly from the central panel, blending into a curved flanged portion. In one prior art method the blank is formed to include a groove around the central panel inward from the chuck wall. This initial blank is then subjected to a curling operation to form a curled edge on the flange, the curled edge being turned somewhat under the flanged portion.
From the curling operation, the partially formed shells are fed through further tooling where they are gripped in the flange portion, while the curled edge is protected in the tooling against deformation. If the groove is already in the blank, then the groove may be reformed. If not, the thus clamped blank is moved against a stationary support applied against the major underside of the central panel.
There is an unsupported region in the shell comprising the edge of the central panel which overlaps and extends beyond the stationary support, out to the region where part of the chuck wall is clamped. This collapsing action places the blank in compression, and results in a reshaping of the unsupported band of material between the chuck wall and the central panel, into a shape which defines a reinforcing channel of countersink at the bottom of the chuck wall and into the periphery of the central panel. Thus, the formation of the end shells according to the prior art requires a three stage operation, and the above described formation of a reinforcing channel shape into the shell results from a working of a band of the metal blank between the chuck wall and the central panel which is essentially uncontrolled and thus susceptible to breaks, distortion, or potential thinning of the shell at this critical point in its structure.
In addition, prior art shells are subject to a condition in the region of the peripheral flange and curled edge which is known in the art as "earring". When the blank of metal is severed from the supply strip, usually a strip withdrawn from a roll thereof, prior practice is to cut or sever a round blank, and little attention is given to the grain direction of the metal, which runs lengthwise of the strip. It has been known for some time, but apparently uncorrected, that forming of the metal (particularly thin aluminum) in operations which are intended to produce a round shell, results in some distortion of the shape from the initial round blank, because the metal tends to stretch slightly more with the grain than across the grain, and to stretch even further at 45.degree. to the grain. The result of such uneven "growth" of the metal appears as a slight deformation in the edge of the blank which is subjected to the curling operation. The curled under edge thus is somewhat closer to the chuck wall in certain areas than in others around the shell; i.e. the end curl becomes irregular with respect to the chuck wall.
This situation can result in one of two difficulties. If the shell is manufactured such that the enlarged "earrings" on the periphery form the primary seal in the seam of the end to the can, then the end curl of the blank between the "earrings" is short, and must rely more upon the sealing compound to maintain the hermetic seal since the metal of the end curl may not tuck completely under the curl on the can body in those regions. In terms of describing the completed seam, it can be said that the end or cover hook does not extend completely behind the body hook throughout the seam.
Alternatively, to achieve a hermetic seal between the end and the body, the design may accommodate for the enlargement of the "earrings", such that the edge between such earrings is completely tucked under the body curl during seaming. This, however, leaves an excess of metal in the cover or end hook extending into the seam in the region where the earrings exist, and this can lead to puncturing of the thin can body in the region of the neck, or to wrinkling of the excessive material within the curled seam, thereby destroying the uniformity of the seam. Whatever the result, the tendency is to have an unacceptably great percentage of cans which leak after they have been filled and sealed. This of course is unacceptable from the standpoint that the packaged product is lost, and additional damage from spillage, etc. may also result.